1. Field of the Invention
The present invention relates to ceramic glaze coatings, and more particularly to ceramic glaze coatings for application to low density, fibrous, rigid silica insulation.
2. Description of the Prior Art
In recent years, rigid, fibrous silica insulations have been developed with densities as low as 9 pounds per cubic foot for possible application as a lightweight reusable surface insulating material. Such insulating materials find application, for example, in space shuttle hardware where low density and very low thermal conductivity are requirements for the insulating material. In that the low density, fibrous, refractory, silica insulations are quite porous and readily absorb moisture, it is necessary to coat the surface of such insulations with a coating which renders them impervious to moisture. Also, to be economical for such uses, the insulating materials must be reusable. That is, the insulation must repeatedly withstand the extreme temperature changes of approximately 2,500.degree.F. which are occasioned by re-entry into the atmosphere from space without delamination or spalling of the insulation.
It is known in the art that silicon-containing substrates can be coated with protective ceramic compositions. See, for example, U.S. Pat. Nos. 3,637,425 and 3,458,345. However, such ceramic compositions are inadequate for use in space shuttle applications because they will not withstand repeated severe thermal cycling in the temperature ranges encountered in such applications without delamination. Moreover, such coatings do not provide the high emissivity levels desirable for such applications.
It is an object of the instant invention to provide a ceramic glaze coating for low density, fibrous, rigid silica insulation which overcomes the aforementioned problems. Moreover, it is an object of the present invention to provide a moisture-impervious ceramic glaze coating for rigid, low density, fibrous silica insulations. Further, it is an object of the instant invention to provide a hard ceramic glaze coating with a high emissivity and a low coefficient of thermal expansion which withstands thermal cycling in the range of -250.degree. F. to 2,500.degree. F.